Testing of new potent and safe vaccine vectors is important to development of an HIV vaccine. Vaccine vectors based on attenuated vesicular stomatitis virus (VSV) are potent inducers of both cellular and humoral immunity. Although VSV vectors have shown no pathogenicity in more than 100 non-human primates when given by nasal, oral, or intramuscular routes, approval for clinical trials was difficult because of concerns about potential pathogenesis of any live-attenuated virus vectors. To address such safety concerns, and generate vectors that would be approved more easily, new single-cycle vectors based on VSV and also a hybrid VSV-Semliki Forest Virus (SFV) propagating replicon, have been developed and tested in mice with excellent results. These vectors also have the significant advantage that there is no pre-existing immunity to them in the human population. The major goal this project is to test the effectiveness of these new vectors in non-human primates for induction of both cellular and humoral immunity. The analysis of induction of neutralizing antibody requires that we use a pathogenic SIV model in which neutralizing antibody can be generated. In the first aim of the project, single-cycle, VSV-based priming vectors expressing SIVsmE660 Env and Gag proteins with and without the cytokine GM-CSF will be prepared and characterized. Expression GM-CSF during priming by VSV vectors enhances memory T-cell recall in mice. In addition, VSV-SFV hybrid replicon particles expressing the same Env and Gag proteins will be prepared as boosting vectors. In the second aim, these vectors will be evaluated in rhesus macaques. SIV neutralizing antibody responses and SIV-specific T-cell responses will be studied in detail following prime and boost. We hypothesize that the new vectors will be highly effective at inducing cellular immune responses, and may be able to induce or at least prime for SIV neutralizing antibody when the appropriate Env antigen is expressed. Vaccinated macaques and controls will be challenged with the highly pathogenic SIVsmE660 strain and detailed virological and immunological analyses will be performed following challenge. Use of the SIVsmE660 challenge model has the advantage that significant neutralizing antibodies to the challenge virus are generated during infection of rhesus macaques. If these antibodies can be generated or primed for during vaccination, there is the potential to advance an SIV model of AIDS in which neutralizing antibody in addition to cell mediated immunity, may be able to prevent infection or at least contribute to controlling viral load following infection. PUBLIC HEALTH RELEVANCE: The AIDS epidemic began more than twenty-five years ago and has killed more than 27 million people including 2.9 million in 2006, yet no effective AIDS vaccine has been developed. The goal this project is to test potent, but non-pathogenic, virus-derived vectors for which there is no pre-existing immunity in the human population. The vectors will be evaluated primarily for their effectiveness at inducing SIV neutralizing antibody and cell mediated immunity, but also for their ability to protect against AIDS in a stringent, non-human primate challenge model. These vectors could later be moved into clinical trials as HIV vaccines using antigens capable of inducing broadly reactive cellular and humoral immunity to HIV.